The long-term goal of the proposed research is to study the transcriptional regulation of the phase II drug-metabolizing and clearing enzymes by the xenobiotic nuclear receptor PXR. The phase II conjugating enzymes such as the UDP-glucuronosyltransferases (UGTs) function in concert with the oxidative phase I cytochrome P450 enzymes (CYPs) to eliminate xenobiotics such as prescription drugs and over-the-counter medications, and endobiotics such as bilirubin, steroid hormones and bile acids. The nuclear receptor PXR was initially identified as a xenosensor to regulate the expression of CYP genes via its binding to the PXR-response elements found within the mammalian CYP gene promoters. The identity of PXR as a species-specific xenosensor has been established by previous transgenic and gene knockout studies. Having established CYP genes as PXR targets, the presence of candidate PXR response elements in genes encoding the phase II UGT gene products raises the potential for a broader physiological function of PXR in xenobiotic response and clearance. However, whether UGTs are induced by PXR is unclear. To investigate UGTs as potential transcriptional targets of PXR, we propose to: (1) clone the UGT1A1 promoter and characterize its regulation by PXR; (2) examine the regulation of hepatic UGTs in mouse models bearing heightened (VP-hPXR transgenic), compromised (PXR knockout), or "humanized" (PXR knockout/hPXR transgenic) receptor activity; (3) examine the effect of altered PXR activity on bilirubin homeostasis; (4) examine the regulation of intestinal UGTs by PXR. If the regulation of phase II enzymes such as the UGTs by PXR proven to be true, we are toward establishing PXR as a master transcriptional regulator of the mammalian xenobiotic response. The results of these studies will provide novel elements in understanding the transcriptional regulation of UGT. It is anticipated that elucidation of the molecular basis of UGT regulation using the "humanized" mice will have great implication in human physiology and diseases. These include bilirubin and hormonal homeostasis, drug metabolism, and chemical carcinogenesis.